#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <climits>
#define int long long 
#define x first 
#define y second 
using namespace std ;
typedef pair<int,int> PII;
const int N = 1e5 + 10;
int a[N];
PII p[N];
void solve()
{
    int n;
    cin >> n;
    for(int i = 1; i <= n; i++) cin >> a[i];
    if(n == 1)
    {
        cout << "-1" << endl;
        return;
    }
    if(n % 2 == 0)
    {
        cout << n / 2 << endl;
        return;
    }
       if(n % 2 == 1)
    {
        cout << n / 2 + 1 << endl;
        return;
    }
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr),cout.tie(nullptr);
    int t = 1;
   // cin >> t;
    while(t -- ) solve();
    return 0;
}